REPORTS is a more relevant variable than nanorod Initial Cds hollow sphere Ag2 S hollow sphere Recovered Cds hollow sphere length in determining the shape change Other even more complex and high-energy nonequilibrium shapes of nanocrystals, such as hollow spheres and branched tetrapods(9, 12) maintain their overall shapes throughout complete cation exchange cycles, provided they have a dimension thicker than 5 nm XsEE9iRes Fig. 3). Cds hollow spheres maintain overall Ag?Te tetrapods morphology during the cation exchange, although a smoothing of the rough surface d a small increase in volume are observed In the case of CdTe tetrapods, slight expan- sion(5%)of the width of each branch is observed after the transformation to Ag te The observed changes in size can be counted for by changes in the crystal ur cell symmetry and lattice parameters during the ig. 3. (A to C) TEM images of (A)initial Cds hollo transformation. In Fig. 4A, the structures of cation exchange of CdS, and(C)recovered CdS from the reverse change reaction. (D to Se2- sublattices in wurtzite CdSe and various TEM images of(D) initial CdTe tetrapods, (E) Ag, Te tetrapods from cation exchange phases of Ag, Se are presented to show the CdTe, and (F)recovered CdTe from the reverse cation exchange topotaxial relationship between the reactant and product phases and associated changes in dimension. Small increases in the width observed in the transformation of thicker cdse rods to tetragonal Ag, Se rods(Fig. 2, F and D) CdSe reflect the changes Ag2Se of the crystal structure as shown in Fig. 4A. Wurtzite Tetragonal Cubic These observations reveal a second fundamen. tal feature of cation exchange in nanocrystal The anion sublattice connectivity is preserved ···中中·:·····中平 during exchange in large nanocrystals 5o品Eg品 There are two possible explanations for the crossover of morphology change at widths of 4 to 5 nm observed in CdSe nanorods. First, the 4“…:# structure of the reaction product is progres- sively changing from a cubic to a tetragonal 4444 phase and eventually adopts an orthorhombic 444 44444,444 phase in the bulk material in crystals of larger 4444·,,· size. For small spheres and thin, short rods of 4444 44 CdSe, like those in Figs. I and 2A, the Ag, 4444a4a product is cubic. For thicker rods such as E and J, the Ag tetragonal (fig. S1). The cubic phase of Ag, Se is a superionic conductor, with a diffusion Wx1.0 15 Wx1.15 coefficient for Ag+ ions similar to that in x1.0 x1.0 liquid solvent(10 cm/s), unlike in other phases of Ag Se(22, 27). Because the smaller CdSe nanocrystals are those that form the reaction zone width a crystal size reaction zone width crystal size cubic phase of Ag, Se and lose structural rigidity, it is conceivable that the high mobility of the Agt ions influences the orphology of the crystal during the reaction. However, we consider this unlikely, because it H●。【圆 is the anion sublattice that forms the structural framework of the crystal in the cation final nitial final exchange reaction, and this should occur Fig. 4.(A) Comparison of the projection of the selenium anion sublat Am山以如 i we anions in different atomic layers inthe direorods. Dark and light colors are used to distinguish the regardless of the degree of cation mobility consider that 4 to 5 nm is comparable to the best of a unit cell is also shown superimposed on the anion sublattice structure to facilitate the available estimates of the width of the reaction arison. Anion sublattices show simple topotaxial relationships, where the transformation phase transformation and microscopic morphol- perpendicular to the long axis(c axis)of CdSe. (B)Illustration of the size-dependent morphology ogy changes in the bulk have been considered product phase, respectively The green region indicates the reaction zone where the structural extensively. The evolution of the reaction front equilibrium is not yet established. www.sciencemag.orgSciEnceVol3065NovemBer2004 1011is a more relevant variable than nanorod length in determining the shape change. Other even more complex and high-energy nonequilibrium shapes of nanocrystals, such as hollow spheres and branched tetrapods (9, 12), maintain their overall shapes throughout complete cation exchange cycles, provided they have a dimension thicker than È5 nm (Fig. 3). CdS hollow spheres maintain overall morphology during the cation exchange, although a smoothing of the rough surface and a small increase in volume are observed. In the case of CdTe tetrapods, slight expan￾sion (È5%) of the width of each branch is observed after the transformation to Ag2Te. The observed changes in size can be accounted for by changes in the crystal unit cell symmetry and lattice parameters during the transformation. In Fig. 4A, the structures of Se2– sublattices in wurtzite CdSe and various phases of Ag2Se are presented to show the topotaxial relationship between the reactant and product phases and associated changes in dimension. Small increases in the width observed in the transformation of thicker CdSe rods to tetragonal Ag2Se rods (Fig. 2, F and J) reflect the changes in dimension upon change of the crystal structure as shown in Fig. 4A. These observations reveal a second fundamen￾tal feature of cation exchange in nanocrystals: The anion sublattice connectivity is preserved during exchange in large nanocrystals. There are two possible explanations for the crossover of morphology change at widths of 4 to 5 nm observed in CdSe nanorods. First, the structure of the reaction product is progres￾sively changing from a cubic to a tetragonal phase and eventually adopts an orthorhombic phase in the bulk material in crystals of larger size. For small spheres and thin, short rods of CdSe, like those in Figs. 1 and 2A, the Ag2Se product is cubic. For thicker rods such as those in Fig. 2, E and J, the Ag2Se is tetragonal (fig. S1). The cubic phase of Ag2Se is a superionic conductor, with a diffusion coefficient for Agþ ions similar to that in liquid solvent (È10–5 cm2/s), unlike in other phases of Ag2Se (22, 27). Because the smaller CdSe nanocrystals are those that form the cubic phase of Ag2Se and lose structural rigidity, it is conceivable that the high mobility of the Agþ ions influences the morphology of the crystal during the reaction. However, we consider this unlikely, because it is the anion sublattice that forms the structural framework of the crystal in the cation exchange reaction, and this should occur regardless of the degree of cation mobility. A more likely explanation arises when we consider that 4 to 5 nm is comparable to the best available estimates of the width of the reaction zone. Solid-state reactions and the associated phase transformation and microscopic morphol￾ogy changes in the bulk have been considered extensively. The evolution of the reaction front Fig. 3. (A to C) TEM images of (A) initial CdS hollow spheres, (B) Ag2S hollow spheres produced from cation exchange of CdS, and (C) recovered CdS from the reverse cation exchange reaction. (D to F) TEM images of (D) initial CdTe tetrapods, (E) Ag2Te tetrapods produced from cation exchange of CdTe, and (F) recovered CdTe from the reverse cation exchange reaction. Fig. 4. (A) Comparison of the projection of the selenium anion sublattice in the wurtzite CdSe nanorod and different phases of Ag2Se nanorods. Dark and light colors are used to distinguish the anions in different atomic layers in the direction corresponding to the long axis of CdSe. Projection of a unit cell is also shown superimposed on the anion sublattice structure to facilitate the comparison. Anion sublattices show simple topotaxial relationships, where the transformation between different structures can be accomplished by movement of ions mostly in the planes perpendicular to the long axis (c axis) of CdSe. (B) Illustration of the size-dependent morphology change during the reaction. Orange and blue colors indicate the regions of initial reactant and final product phase, respectively. The green region indicates the reaction zone where the structural equilibrium is not yet established. 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